The present invention relates to a circuit breaking element, and more particularly to improvements in circuit breaking elements of the super-small type for use in electric circuits of electric devices.
Such circuit breaking elements usually comprise an electrically insulating base member of ceramic or the like and a resistance coating layer of copper, copper alloy or some other metal directly formed over the surface of the base member by plating or vacuum evaporation. These conventional elements have the serious drawback of failing to assure safety because even if excessive current passes through the element to melt the resistance coating, the circuit will not be broken completely.
This drawback is attributable to the structure wherein the metal resistance coating layer formed by plating, vacuum evaporation or like method has a small thickness and is formed over the surface of the insulating base member. With this structure, the space formed by the melting of the metal coating with excessive current is very small, invariably permitting residual resistance to remain and failing to completely break the circuit. Thus the element is unable to perform the important function of breaking the circuit by melting. Table 1 shows the relationship between the breaking current value, the breaking time and the insulation resistance determined by a circuit breaking test with use of an element comprising a ceramic base member and a copper plating layer formed over the base member.
TABLE 1 ______________________________________ Breaking current Breaking time Insulation resistance value (A) (sec.) (M.OMEGA.) ______________________________________ 18 0.012 0.15 15 0.047 0.25 " 0.137 0.25 10 0.568 0.1 " 0.706 0.3 " 0.576 0.6 " 0.340 0.5 8 4.31 0.01 " 0.81 0.3 " 0.756 0.1 6 5.24 0.3 " 1.93 0.1 5 216.37 0.3 " 272.58 0.3 ______________________________________
The table reveals that with the circuit breaking element wherein a metal resistance coating layer is formed directly over the surface of a base member, the insulation resistance is small but the residual resistance is great when the metal coating breaks on melting.